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Mass Spectrometric Tracer Pulse Chromatography J. F. Parcher" and M. I. Selim Chemistry Department, The University of Mississippi, University, Mississippi 386 77
Mass spectrometric tracer pulse (MSTP) chromatography measures vapor-liquid or vapor-solid equilibrium data. The technique is based on normal tracer pulse chromatography, but utlllzes stable Isotopes and a mass specific detection system. The procedure was used to measure partition isotherms of propane and carbon dioxide In n-hexadecane over the range of 25 to 50 O C and 400 to 1200 Torr. The agreement of the MSTP data with the literature data is excellent for the COZ/C18H34system at ail three temperatures. There is a discrepancy in the literature for the C3H,/CleH3, system. The MSTP data are in excellent agreement with one data set and 3-10% lower than two other compilations.
There is a tremendous need for a fast, accurate technique to measure vapor-liquid or vaporsolid equilibrium data over a wide range of temperature and pressure. The usual vapor pressure or equilibrium still methodology is accurate, but time consuming; and there are few data available at temperatures and pressures other than 25 "C and 1 atmosphere. T h e potential applications of gas-liquid and gas-solid chromatography in this area have been recognized for many years, but never fully exploited. Normal elution chromatography, with the solute at infinite dilution, has been used successfully for these measurements, and there are many advantages to this technique. The method, however, is limited to systems which obey Henry's law and to solutes which are significantly retained by the stationary phase. The uncertainty in the determination of the dead time or void volume of the column limits the accuracy of elution data for systems in which the solubility is low, i.e., systems with a Henry's law constant greater than lo3. Several methods have been suggested for measuring equilibrium systems chromatographically at finite concentrations, and these methods have been thoroughly reviewed and evaluated (1-3). Presently, none of these techniques have been used extensively for several reasons. The theory of mass transport through a fixed bed reactor or column is very complex. The differential equations describing a normal chromatographic column cannot be solved analytically ( 4 ) ,and numerical solutions require extensive computer time and facilities. Simplifying assumptions have been used to reduce the mathematical complexity of the problem to reach an analytical solution. These assumptions are aften unrealistic, such as zero pressure drop, constant flow rate, or constant gas phase viscosity. This limits the accuracy and applicability of the simpler frontal techniques. There is yet another chromatographic technique for obtaining finite concentration equilibrium data. This method is tracer pulse chromatography (5-8) and is probably the most commonly used type of frontal chromatography (9-18). In this technique, the solute vapor is introduced into a packed column as the carrier gas or as a component of the carrier gas at a known partial pressure. The column is saturated with the carrier gas so that a vapor-liquid or vapor-solid equilibrium is established. A small elution sample of a radioactive isotope of the solute vapor can be used as a probe to determine the amount of solute vapor which is absorbed or adsorbed on the liquid or solid stationary phase. The number of moles 0003-2700/79/0351-2154$01,00/0
of solute, ni, condensed on or in the stationary phase is directly proportional to the corrected retention time, tRi',of the tagged solute, i (8).
ni = MtR[
(1) M is the mass flow rate (moles/min) of the solute vapor in the column. This result is obtained with no assumptions concerning the shape of the isotherm, effect of diffusion, constancy of the void volume, or heat of sorption. The one necessary assumption in normal tracer pulse theory is that there is no isotope effect on the equilibrium properties of the system. Tracer pulse chromatography does not require special chromatographic instrumentation and the method is simple and elegant. The major disadvantage of the method is the necessary detection and handling of radioactive isotopes in the gas phase. The solutes usually contain 3H or 14C isotopes which must be trapped out of the gas stream. The sample size of the eluted isotopes must be small to avoid perturbation of the equilibrium and these small samples are hard to detect with ionization detectors. Tracer pulse chromatography does not inherently require the use of radioactive isotopes. Any distinguishable isotope, such as I3C or 2H, would be satisfactory with the proper detection system. This paper describes the use of stable isotopes and a mass spectrometric detector to extend and improve the capabilities of tracer pulse chromatography.
EXPERIMENTAL The GC/MS system used for this investigation was a Hewlett-Packard Model 59854. This is a quadrapole instrument with a high pumping capacity and real-time selected ion monitoring capabilities. The gas chromatograph was modified for variable outlet pressure operation by the addition of a microneedle valve (ScientificGlass Engineering Pty. Ltd. Model MNW-100) at the outlet. Subatmospheric pressures were obtained by venting the outlet of the valve to vacuum. The flow sensor on the gas chromatograph was calibrated for each carrier gas over a range of flow rates. The molar flow rate for each experiment was obtained from the sensor output and the calibration factors. The inert gases were all Linde Research grade and the carrier gases were Linde Instrument grade. The specified minimum purities were 99.99% for carbon dioxide and 99.5% for propane. The stable isotope solutes, 2,2-d2propane (98% D)and '% labeled carbon dioxide (90% 13C) were obtained from Merck and Co. The n-hexadecane (Altech Associates) was used as the stationary liquid phase. This was coated on 60/80 mesh Chromosorb-P which had been deactivated with dimethyldichlorosilane (Johns-Manville). The columns were made of 0.25-in. 0.d. copper tubing of various lengths from 200 to 350 cm. The liquid loading for the three columns used in this investigation ranged from 24-32%.
The accurate measurement or calculation of the retention time, truely unretained solute is a critical factor in tracer pulse chromatography. The standard use of an air peak or a single inert gas peak is totally inadequate because all of these gases are soluble in hexadecane to some extent at the temperatures and pressures used in this investigation. Several methods have been proposed for the accurate determination of to. All of these methods are based on some form of linearization scheme for an equation of the form (2) In ( t R i - to) = a + P+i to, of a
0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
2155
Table I. Solubility (Mole Fraction) of Propane and Carbon Dioxide in n-Hexadecane at One Atmosphere King and Chappelow and Hayduk et Lenoir vapor temp,, " C this work Al-Najjar (30) Prausnitz (29) al. (28, 32) et al. ( 3 2 ) propane
25
0.128
0.139*
0.124*
0.137
8.05
carbon dioxide
35
0.104
0.114*
45
0.087
0.095*
0.087*
0.109 0.0896
.__
11.46
25
0.0142
0.142
40
0.0123 0.0114
0.0123* 0.0113
50
0.104* 9.96
0.135 '1.41 .__
___
0.0138
--_
0.0121 0.0113
__-
0.016 62.5
* Interpolated from measured values at other temperatures. a and /3 are constants and $irepresents some extrapolation
parameter for solute i. A flame ionization detector will not respond to an air sample, so chromatographers have developed a method for determining to based on the use of the carbon number of a series of n-alkanes (1g21) or other homologs (22) for Gi.This is a popular method, but is not acceptable for solutes with small tRi- tovalues because Equation 2 is often not valid for the lower members of a homologous series when carbon number is used for the $iparameter. If higher members of a series are used, the procedure involves a long extrapolation and a small uncertainty in the tRivalues causes a large uncertainty in to (22). Other workers have used the inert gas series of solutes and various physical parameters for ICi. Heats of adsorption (23), polarizability (13, 16), the square root of the Lennard-Jones potential (141,and the Kirkwood-Muller potential parameter (17) have all been used for Gi. We found the Kirkwood-Muller potential function (17)tQ give the best straight line fit with nonlinear least squares regression of Equation 2. This technique was used throughout the investigation to determine tobased on the retention of neon, argon, krypton, and xenon. The mass spectrometric detection system has the advantage over ionization chambers or conventional detectors that the chromatograph can be operated a t low pressures ( 2 4 ) with no deleterious effects on the sensitivity of the detection system. The effect of reduced pressure on the efficiency of the chromatographic column is less certain. Shellier and Guiochon (25)concluded that vacuum outlet operation had little or no effect on the plate height. Hatch and Parrish (24) showed that under other conditions there is a large effect on the efficiency. In general, atmospheric pressure operation yielded a lower plate height, but the optimum flow rate was always larger for the vacuum conditions. The effect of column pressure on the solute plate height was not systematically investigated; however, there was no significant loss of efficiency observed a t subatmospheric pressures. The use of a mass specific detection system obviates the assumption that there is no isotope effect on the equilibrium measurements. In our studies, we found that there was no measurable difference in the solubilities of C 0 2 and I3CO2 in hexadecane. On the other hand, there is a significant difference between the solubility of natural propane and dideuterated propane in hexadecane. The heavier isotope is less soluble by 1-2%. This type and magnitude of an isotope effect has been observed previously (26,27)and must be taken into account for accurate measurements by tracer pulse chromatography. The relative retention times of eluted samples of natural and isotopic solutes were used as correction factors to determine the retention time of a natural solute from the observed retention time of a isotopic solute. R E S U L T S AND D I S C U S S I O N The experimental isotherms for propane and carbon dioxide in n-hexadecane are shown in Figures 1 and 2. Comparison of the experimental data with previous nonchromatographic studies a t a single pressure (1atm) are given in Table I, where the data marked with * are interpolated from measured values at other temperatures. In some cases, the experimental results were presented as Henry's law constants and these values are given in the second row.
16rc
0
2
14-
X
1
d ! I '
200
400
600
800
1000
1200
Pressure (torr) Flgure 1. Partition isotherms for propane in n-hexadecane at 25,35, and 45 OC. Literature data at 760 Torr. ( * ) Hayduk et al. (28, 31). (A) King and AI-Najjar (30). (0)Chappelow and Prausnitz (29)
T h e M S T P data for carbon dioxide agree well with the literature values except for the data of Lenoir e t al. (32). This measurement was carried out by elution chromatography at the limit of P = 0 and the extrapolation t o atmospheric pressure may be questionable, although our data over a range of pressure indicate t h a t the Henry's law constant is independent of pressure u p t o 1200 Torr. The propane measurements agree with those of Chappelow and Prausnitz (29)within 2% but are 3-10% lower than the other literature values (26, 30-32). T h e data reported here were all measured at a constant molar flow rate with variable inlet and outlet pressures and volume flow rate. Extensive measurements were also carried out with a constant outlet pressure and variable flow rates and inlet pressure. There was no measurable difference in data obtained under these two sets of conditions. The liquid mole fraction of propane was a function of the mean pressure of propane in t h e column and the temperature, and was independent of flow rate, inlet and outlet pressures, and the amount of hexadecane in the column. M S T P chromatography can be used t o measure solubility data with accuracy comparable t o established methods and
2156
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
I
/ /
18
16
c
i # oOLZOO
I
o'/' /
n
/
1
O
~
do
600
800
1000 I200
Pressure (torr) Figure 2. Partition isotherms for carbon dioxide in n-hexadecane at 25, 40, and 50 O C . Literature data at 760 Torr is the same as Figure
1
has all the advantages of normal chromatographic techniques, such as speed, simplicity, and wide temperature and pressure ranges. T h e use of a mass specific detection system allows the use of nonradioactive isotopes, subatmospheric pressures, and quantitative evaluation of any possible isotope effects. One of the significant disadvantages of the static volumetric or gravimetric techniques is the excessively long times required to ensure complete equilibration in the system. A commonly quoted figure is 2-4 h (28, 29) and the procedures require extensive degassing of the solvent. In a chromatographic column the solvent is present as a very thin film spread over a large surface area and all of the solvent is continuously in contact with the solute vapor, so the equilibration time is essentially the time required for the solute front to pass through the column. In one experiment, the retention time of a tagged sample of propane was measured at intervals of 10-15 min for the system of propane/hexadecane a t 45 "C. T h e retention time was constant within 2% for the time interval from 5 to 110 min. The required equilibration time varied with flow rate and pressure but was always less than 10 min after the vapor front reached the outlet of the column. The major disadvantage of the procedure is the requirement of a mass spectrometer as a mass specific detector. This is complex and expensive instrumentation; however, the simplest form of quadrapole mass spectrometer is adequate for this procedure. The experiment does not require modification of the mass spectrometer and can be used on any GC/MS system. As with any chromatographic technique, the solvents are limited to those with low vapor pressure a t the operating
temperatures. This is a restriction; however, the technique is applicable to high molecular weight solvents and polymers, and these are precisely the types of systems which are not amenable to other techniques. The tracer pulse method requires large amounts of vapor for the carrier gas. This can be introduced as the carrier alone or as a component of a mixture of gases. The solute can be used alone as the carrier if the vapor pressure is sufficient at room temperature. The solute vapor can also be introduced by bubbling an inert gas through a saturator containing liquid solute, or the solute can be pumped into the system as a liquid and vaporized ahead of the column. The equilibrium data obtained by this procedure or any chromatographic method, must be an average value. The inlet of a column must be at a higher pressure than the outlet and the solubility can only be given at the mean column pressure. Probably the most significant potential for the technique is in the study of multicomponent systems. The mass specific detection system can differentiate components of a mixture even if the chromatographic resolution is inadequate for a separation. Vapor-liquid equilibrium data for multicomponent systems cannot be obtained readily by any other nonchromatographic technique.
LITERATURE CITED Laub, R. J.; Pecsok, R. J. "Physiochemicai Applications of Gas Chromatography", Wiiey Interscience: New York, 1978. Parcher, J. F. In "Advances in Chromatography", GMdings, J. C., et ai., Eds.; Marcel Dekker: New York, 1978; Voi. 16, Chapter 5. Locke, D. C. I n "Advances in Chromatography", Giddings, J. C., et ai., Eds.; Marcel Dekker: New York, 1976; Voi. 14, Chapter 4. Parcher, J. F.; Ho, T. H.; Haynes, H. W. J. Phy. Chem. 1976, 80, 2656-2661. Heifferich, F.; Peterson, D. L. Science. 1963, 742, 661-663. Staikup, F. I.;Deans, H. A. AIChE J. 1963, 9 , 106-108. Giimer, H. B.; Kobayashi, R. AIChE J. 1965, 7 1 . 702-705. Peterson, D. L.; Helfferich, F. J. Phys. Chem. 1965, 69, 1283-1293. Asano, K.; Nakahara, T.; Kobayashi, R. J. Chem. Eng. Data 1971, 16, 16-18. Peterson, D. L.; Heifferich, F.; Carr, R. J. AIChE J . 1966, 72, 903-905. Haydel, J. J.; Kobayashi, R. Ind. Eng. Chem., fundsm. 1967, 6,546-554. Masukawa, S.;Kobayashi, R. J. Chem. Eng. Data. 1968, 73, 197-199. Masukawa, S.; Kobayashi, R. J. Gas Chromatogr. 1968, 6 , 461-465. Hori, Y.; Kobayashi, R. J. Chem. Phys. 1971, 54, 1226-1236. Khoury, F.; Robinson, D. B. J. Chromatogr. Sci. 1972, 70, 683-690. Kobayashi, R. J. Gas Chromatogr., 1968, Masukawa, S.;Alyea, J. I.; 6 , 266-269. Nakahara, T.; Chappelear, P. S.;Kobayashi, R. Ind. Erg. Chem., Fundsm. 1977, 16, 220-228. Everett, A.; Kobayashi, R. AIChE J. 1978, 24, 745-747. Guardino, X.; Aibaiges, J.; Firpo, G.; Rodriquez-Vinals, R.; Gassiot, M. J . ChrOmatwr. 1976, 778, 13-22. Garcia DMninguex, J. A.; Garcia Munoz, J.; Femandez Sanchez, E.; Mdera, M. J. J. Chromatogr. Sci. 1977, 75,520-527. Haken, J. K.; Wainwright, M. S.;Smith, R. J. J. Chromatcgr. 1977, 733, 1-6. Ashes, J. R.; Mils, S.C.; Men, J. K. J. chromatog. 1978, 766,391-396. Masukawa, S.; Kobayashi, R. J. Gas Chromatogr. 1966, 6, 257-265. Hatch, F. W.; Parrish, M. E. Anal. Chem. 1978, 50, 1164-1172. Sellier, N.; Guiochon, G. J. Chromatogr. Sci. 1970, 8, 147-150. Bruner, F.; Cartoni, G. P.; Liberti, A. Anal. Chem. 1966, 38, 298-303. Cartoni, G. P.; Liberti, A,; Peb, A. Anal. Chem. 1967, 39, 1618-1622. Hayduk. W.; Walter, E. B.; Simpson, P. J. Chem. Eng. Data 1972, 17, 59-61. Chappelow, C. C.; Prausnitz, J. M. AIChE J . 1974, 20, 1097-1104. King, M. 5.;Ai-Najjar, H. Chem. Eng. Sci. 1977, 32, 1241-1248. Hayduk, W.; Castaneda, R. Can. J. Chem. Eng. 1973, 57,353-358. Lenoir. J. R.; Renauit, P.; Renon, H. J. Chem. Eng. Data 1971, 16, 340-342.
RECEIVED for review June 5 , 1979. Accepted August 13, 1979. This work was supported by grant number CHE-7809918 from the National Science Foundation.
